In the manufacture of semiconductor devices, it almost universally necessary to apply a layer of some type of material overlying the entire device, excluding the bonding pads. In high density memory devices, it is often necessary to apply a layer of alpha particle protection material. In mesa type devices it is necessary to passivate the exposed junctions at the edges of the mesa structure. In nearly all semiconductor devices, it is necessary to apply a passivation layer to electrically and mechanically protect the circuit.
Prior art solutions to the problem of applying passivation and other materials to semiconductor devices the many and varied. Edge passivation of mesa type devices is typically accomplished by the application of a photosensitive mixture which includes small glass particles. The material is patterned using conventional photolithography and a combination of baking and firing steps transform the remaining material to a passivating glass. Application of alpha particle protection and passivation materials is commonly accomplished by spinning on a liquid which is then dried or cured in some fashion to produce the desired layer. Spin on techniques are commonly limited to a few tens of thousands of Angstroms in the thickness of the layer of material which may be applied. Thicknesses in this range are commonly unacceptable for alpha protection purposes. In addition, a spun on layer must be subjected to some type of patterning to open the passivation or alpha protection material over the bonding pads.
Recently, screen printing mechanisms have come into use in the electronics industry and have been applied to printed circuit board, liquid crystal display and other coating processes. Typically, a photosensitive emulsion supported on a stainless steel mesh is patterned to protect areas of the surface being printed which are not to receive a coating of the printed material. The mask is then aligned over the board, display or other product and the material to be applied is forced through the mesh where the photosensitive emulsion has been removed. It is possible, using such a technique, to apply layer having a thickness of several mils.
A screen printing approach would be desirable for the application of passivation and other coatings to semiconductor wafers. However, it has been found that the photosensitive emulsion, even after having been developed and fixed, does not resist the solvents used in certain desirable passivation and alpha protection materials. Thus, a screen used in such a printing process must be replaced very frequently. Up to now, attempts to improve the longevity of screens used for the application of passivation and alpha protection materials to semiconductor wafers have not been successful.